Abstract

The use of named formatting properties to influence the
presentation of documents on the web has become widespread. As XML
vocabularies are now being combined in many ways, it is
imperative that a common set of property names, values, and
semantics be maintained.

This finding establish an architectural principle
to that effect.

Status of this Document

This document has been developed for discussion by the
W3C Technical Architecture Group.

This document is the work of the editor. It is a draft
with no official standing. It does not necessarily represent the
consensus opinion of the TAG.

Table of Contents

1 Formatting Properties on the Web

The use of named formatting properties to influence the
presentation of documents on the web has grown from its introduction
in [CSS1] to include a wide range of specifications:
[CSS2], [SVG], [SMIL],
[MathML], [XSL], and possibly others.

This has a positive benefit for the user- and developer-communities
because it reduces the number of property languages that need to be
understood by users and applications. Furthermore, as XML vocabularies
are now being combined in many ways, it is becoming more than merely
beneficial, it is becoming imperative that a common set of properties
and values be developed.

Consider an application that is expected to render a compound
document that consists of XSL Formatting Objects, XHTML table markup,
SVG diagrams, and MathML equations. Typically, the author (and the
reader) would want consistent styling for all these pieces.

Because many properties are considered "inheritable",
each of the vocabularies must use the same properties for the same
purpose in order for the styling to be consistent. In addition to
using the same properties, the interpretation of those properties must
be the same in each vocabulary.

Consider a simple concrete example, where only two vocabularies are nested:

In the absence of any other direction (from an external stylesheet,
for example), one imagines that selection of a font family and size on
the HTML div element applies to both the the HTML text and
the SVG text.

It would be architecturally unsound to suggest that different
vocabularies should specify the same or very similar traits using
different property names or values. In this particular case, for example,
it would be unsound to suggest that the "font-family" property
be used to influence HTML text but the "fontFamily" property
be used to exert the same influence over SVG.

2 A History of Successful Coordination

The CSS, HTML, SVG, and XSL Working Groups have a shared history of
successful coordination on formatting properties and values. It is
possible that specific actions could be taken to assure continued
coordation: establishing key liasons, formation of a coordination
group, or even broad reorganization of the specifications.

However, these actions are outside the scope of this finding which aims
simply to specify an architectural principal.

3 An Architectural Principle

Formatting property names, values, and semantics must be consistent
across all specifications. Whenever a working group suggests the
creation of a new formatting property, or the addition of, or a change
to, an existing formatting property's allowed values, the working
group must show a strong justification for not using an existing
formatting property or properties that are related to the proposed new
property or value.

Clearly innovation on the web will create situations where new
properties are required and existing properties will need to be
extended. What we must avoid doing is changing the semantics of
existing properties in ways that introduce unnecessary
interoperability issues.